S-adenosylmethionine synthetase (EC 2.5.1.6) catalyzes the synthesis of S-adenosylmethionine (S-Adenosyl Methionine, SAM or SAMe) from adenosine triphosphate (ATP) and methionine. SAM exists in all biological cells. SAM has two configurations: (S,S) and (R,S), but only configuration (S,S) is biologically active. S-adenosylmethionine supplies methyl group to proteins, lipids, nucleic acids and vitamin B12 in vivo and involves in more than 100 biosyntheses and metabolic reactions in vivo in the synthesis of hormones, neurotransmitters, nucleic acids, proteins and phospholipids, and is essential for the normal function of cell membrane and human metabolism and health (Chiang P K, et al. FASEB J 1996, 10:471-480; Bottiglieri T. Am J Clin Nutr. 2002, 76:1151 S-1157S). SAM is also human body's scavenger and antidote (Mato J M. Pharmacol Ther. 1997, 73:265-280; Lu S C. Gastroenterology. 1998, 114:403-407). The amount of SAM in the body decreases as human ages. So the supplement of SAM in human is very beneficial to health. SAM is regarded as a natural food supplement in US since late 1990s.
SAM can be produced through an in vitro enzyme-catalyzed transformation process, which catalyzes the synthesis of SAM from precursor of methionine and adenosine triphosphate (ATP) using bacterium-expressed S-adenosylmethionine synthetase in vitro as described by Zhang (patent publication No: CN1483829, 2004). In the patent CN1483829, the synthesis of SAM from the precursor of methionine and adenosine triphosphate (ATP) must be conducted at 30-35° C. In addition, because the S-adenosylmethionine synthetase used is a crude extract, it contains substantive other enzymes that degrade ATP and methionine, leading to heavy consumption of expensive ATP precursors and methionine, thereby increases the cost of SAM production. Furthermore, the existence of other enzymes in the crude extract of SAM results in the presence of many contaminations in the crude product and thus increases the cost of separation and purification.
Therefore, increasing the activity of S-adenosylmethionine synthetase, shortening the reaction period and using purer S-adenosylmethionine synthetase to prevent precursor degradation are crucial to reduce the cost of SAM production.